Oceanic core complexes provide an accessible window into deep processes occurring at slow and ultra-slow-spreading mid-ocean ridges. We analyse samples from IODP ocean drilling of core complexes at the Atlantis Bank, Atlantis Massif, and near the Kane Transform at the South West Indian and Mid-Atlantic ridges. We correlate secondary minerals, including oxides, with sites of melt migration. We interpret changes to mineral assemblage and microchemistry, reaction textures and melt-pseudomorph microstructures as fingerprints of open-system melt-mediated processes. This micro-scale information is combined with a macro-scale review of legacy mineral chemistry data to show that melt-fluxed rocks share remarkably similar characteristics across the three core complexes investigated. These are rich in oxides and have olivine, orthopyroxene and clinopyroxene grains that are chemically distinct from oxide-poor gabbros. We propose that oceanic crust fluxed with external melt can be recognised by the following key features: (1) high modes of secondary minerals, such as oxides and olivine, (2) microstructural evidence for the former presence of melt, and (3) mineral chemistry differences between primary and secondary olivine, orthopyroxene, clinopyroxene and plagioclase. Importantly, olivine has previously only been reported as primary, that is, having crystallised from magma. However, our results show that gabbros with secondary olivine are reliable indictors of melt–rock interaction during deformation-assisted diffuse melt migration through the gabbroic oceanic crust. Finally, we propose a new model for the formation of oceanic core complexes where deformation-assisted melt migration plays a critical role in strain localisation, exhumation and evolution of the core complex.

The Strathbogie Complex is a large granitic body in north central Victoria. Previous detailed mapping of part of its western and southern boundary suggested that the body was relatively thin, probably less than 1 km. However, forward modelling its gravity response suggests the complex is thicker, perhaps more than 6 km in its deepest parts. The only way the measured gravity response could be modelled to fit a maximum thickness of 1.5 km is with an unrealistically low density. While not definitive, the modelling provides a warning that, where available, even excellent surface mapping should be combined with other data to obtain the most realistic subsurface geometries.

Recent studies of two-phase non-coaxial analogue experiments and natural rifts suggest that the pre-existing faults that form in the first-phase rift could strongly influence the fault development during a subsequent phase of extension. However, related models from natural examples are still lacking. Here we compare the fault geometry and evolution with different pre-existing fault arrays in two adjacent areas (Xijiang and Lufeng sags) from the Pearl River Mouth Basin in the South China Sea. This basin has experienced two-phase rifting including the middle Eocene rift phase 1 (NW–SE extension) and a late Eocene–early Oligocene rift phase 2 (N–S extension). The Xijiang Sag developed an approximately NE-striking listric fault system, while the Lufeng Sag formed complex structure is composed of six major faults with various orientations. We demonstrate that the first-phase fault network in the Xijiang Sag is a colinear listric fault system, while that in the Lufeng Sag comprises two sets of (non-colinear) faults. In the Xijiang Sag, the second-phase fault network consists of the reactivated first-phase faults, newly formed faults abutting against or cross-cutting the pre-existing reactivated faults or occurring between the pre-existing faults. In contrast, in the Lufeng Sag, the second-phase faults include partially reactivated first-phase non-colinear faults and new non-colinear faults. The influence of first-phase faults on second-phase faults is manifested in several distinctive ways. Two sets of major non-colinear faults led to the development of more complicated oblique faults in the Lufeng Sag. The development of the second-phase faults is the result of a combination of geometry and heterogeneity of pre-existing fabrics, variation of extension direction and local perturbations set up by the geometry of pre-existing faults.

Small-scale continental basaltic fields can erupt with little warning and bring deep undegassed magmas to the surface rapidly. To explore the lifetime, petrogenesis and plumbing system architecture feeding such basaltic lava fields and compare them with large-scale shield volcanoes, we have focused on the Hoy lava field, central Queensland, Australia. 40Ar/39Ar geochronology, elemental and isotopic whole-rock geochemistry and mineral chemistry on selected Hoy samples reveal long-lived volcanism of ca 50 Ma and magma storage at mantle depths, notably different from the comparatively short duration (3–5 Myr) and crustal magma storage depths of shield volcanoes. In this study, four Hoy lava-field eruptive intervals spanning ca 50 Ma were investigated: 67.5 ± 0.3 Ma, 32.3 ± 0.6–31.6 ± 0.7 Ma, 21.9 ± 0.5 Ma and 18.1 ± 0.3 Ma. In all four eruptive events, samples are porphyritic alkali basalts and trachybasalts (11.41–6.45 wt% MgO) with incompatible element concentrations and Sr–Nd–Pb isotope ratios dominantly derived from a metasomatised sub-continental lithospheric mantle (SCLM) source with an enriched mantle I (EMI) signature. Complex crystal populations show major- and trace-element variations reflecting fractional crystallisation, magma recharge, magma mixing and mantle xenocryst entrainment. Clinopyroxene–melt thermobarometry indicates magma storage in SCLM reservoirs at ∼30–47 km depths. The nearby larger but shorter-lived (3–5 Myr) Buckland central volcano has similar source compositions; however, magma storage is limited and concentrated in the crust, resulting in increased crustal contamination. The results suggest that basaltic centres of contrasting scale and longevity are linked to distinct magma production mechanisms, fluxes, ascent and differentiation.

The Gumants River, rising on the eastern flank of the Mount Hagen stratovolcano, forms part of the headwater catchment of the Wahgi River in the central highlands of Papua New Guinea. The late Quaternary history of the catchment was shaped by a massive debris avalanche with an estimated volume >15 km3 that most likely occurred 400 ± 100 ka. The Upper Gumants River, which occupies about one-quarter of the 420 km2 catchment and underlain almost entirely by volcanic deposits including the avalanche, lahars and widespread airfall tephras that ceased falling ca 200 ka, once flowed directly to the Wahgi River through the Guga subcatchment rather than through the rest of the Gumants Basin. The Kuk Swamp part of the Guga catchment, intensively investigated for evidence of agriculture stretching back 10 ka, was added to the UNESCO World Heritage List in 2008. Nearly 40% of the 79 km2 of the Kugimi Basin is underlain by at least 30 m of swamp deposits with all but the uppermost few metres of the varied swamp and alluvial deposits beyond the range of radiocarbon dating. The Muga Basin forms nearly half of the Gumants catchment with about 50 km2 of swamp deposits and colluvial/alluvial sediments, which have been accumulating rapidly in the last few hundred years but are still underlain at a depth of a few metres by ancient sediments and, by inference, debris avalanche deposits that extend downvalley beyond the junction of the Gumants River with the Wahgi River. While there has been speculation for decades that the Wahgi River once flowed westward, it is now clear that the Gumants River has always flowed eastward, and it seems very likely that that is also true of the Wahgi River.

Copper–gold porphyry deposits are the world’s main source of copper and a significant source of gold. They consist of vein networks and their surrounding alteration zones. Commonly the deposits are centred on narrow intrusions (stocks), but calling these deposits ‘porphyries’ is unjustified because the name carries little descriptive or genetic value. Extensional veins were formed by hydraulic fracturing of the stocks, at depths where open spaces could not be maintained and where fluid pressure approaches lithostatic pressure. The post-crystallisation timing of the veins is important because it indicates that the host stocks could not have been the direct sources of either metals or ore-forming fluids. In the traditional magmatic model, precursor batholiths, lying at depth, are inferred to be the sources of the Cu and Au in the overlying host stocks. In this model, the batholiths are assumed to have crystallised and produced the mineralising aqueous fluids, Cu and Au. However, in many porphyry deposits, the concept of metal and fluid supply from deeper batholiths is problematic. Neither Cu nor Au is strongly enriched during the crystallisation of silicate magmas, and although hypersaline fluids are a characteristic of Cu–Au porphyry deposits globally, the source of the Cl remains unconstrained. There is little evidence that silicate magmas can release such Cl-rich fluids, and it remains unexplained how elevated levels of Cl may be achieved in a silicate magma. Therefore, the starting assumption that these deposits formed predominantly from magmatic sources and processes is questioned. This study has selectively focused on the roles of rheology, rock mechanics, vein control, metal-enrichment processes and the sources of Cl. Non-magmatic processes may be enough to facilitate strong partitioning of Cu and Au into high-temperature, oxidising, high-salinity, hydrothermal fluids to form Cu–Au porphyry deposits.

The Clarke River Fault in northeast Queensland records an early Paleozoic history of subduction, accretion and continental suturing. Samples of mafic–ultramafic rocks collected proximal to the Clarke River Fault record oceanic geochemical affinities and comprise alteration assemblages consistent with an ophiolitic origin. The ca 456 Ma Falls Creek Tonalite records a continental-arc geochemical signature and was formed in response to long-lived subduction beneath the Thomson Orogen. Ordovician subduction beneath the Thomson Orogen is broadly coeval with arc magmatism documented in the Lachlan Orogen, which has been associated with the formation of several large porphyry ore deposits. The Falls Creek Tonalite yields adakite-like geochemical signatures that reflect a fertile melt source conducive to the formation of porphyry ore deposits. The outcropping plutons record ductile deformation consistent with mid-crustal depths, and they were emplaced during late syntectonic activity. This implies that the Falls Creek Tonalite was emplaced at too great a depth to have formed porphyry ore deposits. The northern Charters Towers Province shares many geological similarities to the Greenvale Province, where the erosional level may be shallower, and the potential for porphyry deposit formation and preservation may be greater.

The middle Miocene Fatha Formation is a prominent cap rock for hydrocarbon accumulations in Iraq’s oil fields. It is widely distributed in the Low Folded Zone but less common in the High Folded Zone of Iraq. For the first time, a nannostratigraphic study has been conducted on the formation at the Miryas locality in the High Folded Zone of the Qaradagh Mountain Series, southwest of Sulaimaniyah, Kurdistan Region, Iraq. The Fatha Formation consists of claystone, siltstone, marlstone, gypsum and few limestone beds. Ten samples were taken from marl-rich strata in the upper part of the formation between the red claystones of the Miryas. On the basis of the identified calcareous nannofossils, three biozones are recorded from the upper part of the Fatha Formation: the NN1, NN2 and NN3 zones. The combined stratigraphic ranges of the calcareous nannofossils identified here support the early Miocene–Aquitanian age; Discoaster druggii, Sphenolithus belemnos, S. cometa, S. procerus, S. tintinnabulum, S. delphix and S. capricornutus are marker species of the Aquitanian that have been identified inside the Miryas’s smear slides. These genera/species are also accompanied by the occurrences of Sphenolithus moriformis, Cyclicargolithus floridanus, Reticulofenestra minuta, R. bisecta, S. minuta, S. dictyoda, Coccolithus pelagicus, Helicosphaera carteri and Coronocyclus nitescens. The appearance of Discoaster saundersi at the top of the formation indicates the last occurrence of the NN2 zone of the Aquitanian and first occurrence of the NN3 zone of the Burdigalian. Through this study, for the first time, the early Miocene biozones determined within the Fatha Formation indicate that lower Miocene sediments (facies) were not deposited at some locations in the Qaradagh Mountain Series. Instead, the equivalent cap rock sediments of the Fatha Formation were deposited in the High Folded Zone; this clarifies why accumulation of hydrocarbons is rare within the late Oligocene–earliest Miocene succession in the High Folded Zone rather than the reservoir rocks of the same age in the Low Folded Zone (such as Kirkuk Oil Fields). KEY POINTS First study of nannostratigraphy of the Fatha Formation in the High Folded Zone of Iraq. First study of early Miocene nanno-biozones within the Fatha Formation. First record of NN1, NN2 and NN3 zones within the Fatha Formation.